This invention relates to improvements in electronic circuitry used in moving read/write heads in memory disc drives for use with computers, and, more particularly, to improvements in such circuitry for providing drive signals to voice coil motors for such disc drives.
Voice coil motors are linear actuators that are widely used for moving read/write heads and their support assemblies across discs in computer system disc drives in order to read data from or write data to the disc. Voice coil motors also remove the head from the areas of the disc that store data when the disc drive is turned off and deploy the head onto the disc when the disc drive is turned on. The head floats across the disc surface on a cushion of air resulting from rotation of the disc. In a conventional disc drive, the disc is roughened on at least portions of the disc surface to obviate sticking of the head to the disc surface as the disc is spun from a stop to an operating speed.
As data densities on the discs have increased, the need for greater precision and accuracy in head positioning has also increased. Additionally, spacings between the heads and the discs have decreased to a point where the roughening of the disc surface is impractical. As a result of these changes, a prior art practice of xe2x80x9cparkingxe2x80x9d the head in the innermost data track no longer provides adequate safeguarding of the head or of the disc when the computer system is not in use.
In increasing numbers of disc drives, the head is parked by causing the head and support assembly to traverse a ramp to remove the head from proximity to the disc when the disc drive is deactivated as the system is shut down. When the support assembly for the head reaches the end of the ramp, the head is latched into a storage position. The head then cannot collide with the disc if the disc drive is jarred or bumped, avoiding one potential source of damage to the head or to the disc.
As the system is reactivated, the head is unparked by releasing the latch in response to a UNPARK HEAD command. The head and support assembly then traverse the ramp towards the disc in response to signals delivered to the voice coil motor from a controller. The head must be moving with the correct speed when the support assembly arrives at the end of the ramp in order to be maintained in proximity to the disc without collision between the head and the disc. As a result, the controller must provide drive signals to the voice coil motor resulting in the correct speed for the head when the support assembly for the head exits the ramp.
One method for driving a voice coil motor includes applying a constant voltage to a voice coil in the voice coil motor. However, the voice coil motor generates a back electromotive force (BEMF) because the voice coil is moving in a magnetic field. The actual voltage driving the voice coil motor thus is the sum of the applied voltage and the BEMF, which varies with voice coil motor velocity vM. As a result, the applied voltage is not the actual voltage driving the voice coil motor.
FIG. 1 is a simplified schematic diagram of a driving circuit 10 and voice coil 11 of a voice coil motor in a disc drive, in accordance with the prior art. The voice coil has a first terminal 11xe2x80x2 and a second terminal 11xe2x80x3. The driving circuit 10 includes a current sensing resistor 12 having a resistance RSENSE and coupled to the voice coil 11. A voltage across terminals of the current sensing resistor 12 is proportional to the current through the voice coil 11 which is driven by power amplifiers 13, 14 and 15 having a gain AP, as is explained below in more detail. The voltage across the current sensing resistor 12 is sensed by a sense amplifier 16 having a gain AS to provide a feedback signal. The feedback signal is added to an analog control signal VIN and the resulting voltage is then compared to a reference voltage VREF by an input amplifier 17 having a gain AE that is determined by resistor 18 and RC network 19. An output signal from the input amplifier 17 is then applied to the voice coil 11 by the drive amplifiers 13-15.
Conventional voice coil motor controller circuits employ a digital to analog converter circuit (not shown) for providing the analog control signal VIN to the driving circuit 10 in response to digitally preprogrammed profiles. However, these voice coil motor controller circuits have limited ability to compensate for effects due to wearing of the ramp and of those portions of the head supporting assembly that are in contact with the ramp. Additionally, the feedback provided by the driving circuit 10 does not compensate for voltage errors in the voltage actually present in the voice coil 11 that result from a dc resistance RMOTOR of the voice coil 11.
In one aspect, the present invention includes a voice coil motor driver circuit providing feedback from a voice coil motor coupled to the driver circuit in a disc drive. The voice coil motor driver circuit provides positive feedback having a first loop gain in a head parking or unparking mode of operation and provides negative feedback together with a second loop gain in other modes of operation. The voice coil motor driver circuit is thus able to compensate for errors in a voltage driving the voice coil motor resulting from a dc resistance of a voice coil in the voice coil motor. The voice coil motor driving circuit is also able to compensate for changes in the voice coil motor properties with time or with temperature changes and for effects caused by motion of the disc drive. As a result, head velocity is more precisely controlled during a head unparking operation, reducing probability of damage to the head or disc.